1. Field of the Invention
The present invention is directed to a system for recovery of ethylene from the inert gas purge stream from a vapor-phase production process, in particular, the vapor phase production process for vinyl acetate.
2. The Related Art
Although the invention is explained in terms of vapor phase production of vinyl acetate, the inventive process may be employed wherever an ethylene recovery loop process exists. For example, ethylene oxide/ethylene glycol production or production of acrylates.
Ethylene is a commodity chemical used in various chemical processes for making numerous other chemicals. Ethylene is a particularly important reactant in the vapor-phase production of vinyl acetate. Because ethylene is costly, producers of vinyl acetate by vapor-phase processes find that recovery and recycling of ethylene is an important cost saving measure.
In a vapor-phase vinyl acetate process, inert gases, particularly nitrogen and argon, are vented from the vinyl acetate reactor loop. These gases are introduced with the feed oxygen and in seal purges throughout the unit. Because ethylene is contained in the purge stream, this purge of the inert gases may result in an efficiency loss, controlled by impurities in gases, raw materials, etc, of from about 1% to about 4% of the total ethylene used in the process. In some plants, this inert gas purge stream is simply burned either in a flare or some other device to recover the energy. In these cases, the producer merely accepts the loss rather than attempt to recover the ethylene.
In other plants, the producer may attempt to recover the ethylene. A known way of recovering ethylene is by absorption of the ethylene into vinyl acetate at system pressure followed by depressurizing the absorber residue to recover the absorbed ethylene. Typically, this requires a flash tank in which the pressure can be quickly and dramatically reduced, thus allowing the ethylene to be separated from the vinyl acetate. Once separated from the vinyl acetate, however, the ethylene must be repressurized through the use of a compressor to recover the ethylene and force it back into the reaction loop.
This method requires the use of certain equipment, namely a flash tank and a compressor which, in turn, requires additional equipment and energy cost. Consequently, a method of recovering ethylene requiring less in the way of equipment and energy expenditures remains of interest.
The method of the present invention eliminates the necessity of a depressurizing step followed by a repressurizing step in order to recover absorbed ethylene in the vinyl acetate process. In the method of the present invention, the ethylene in the inert gas purge stream is absorbed in a stream of acetic acid at system pressure with the absorber residue being fed back into the vinyl acetate reactor loop in either the vaporizer or the recycle gas scrubber. In this way, neither a flash tank nor a compressor is needed.
In accordance with one aspect of the invention, a method for the recovery of ethylene from an inert gas purge stream from a reactor loop in a vapor-phase process for making vinyl acetate is provided. A method of the present invention includes the steps of contacting the inert gas purge stream containing ethylene with acetic acid in an absorption vessel; discharging a stream containing acetic acid and ethylene from the absorption vessel; separating the ethylene from the acetic acid in the stream by contacting the stream with ethylene gas in a scrubber column; and recovering ethylene from a top portion of the scrubber column. The method also includes the step of recycling the recovered ethylene to the reactor loop for further use.
An alternate method of the present invention includes the steps of: contacting the inert gas purge stream containing ethylene with acetic acid in an absorption vessel; discharging a stream containing acetic acid and ethylene from the absorption vessel; and conveying the stream to a vaporizer in the reactor loop for further use.